150 years and counting

I was glad to see an Angewandte essay commemorating Kekule’s idea about the structure of benzene (http://onlinelibrary.wiley.com/doi/10.1002/anie.201408034/abstract?campaign=woletoc). This powerful abstraction has been with us for more than 150 years and its foundational nature had invited all manner of follow-up studies that attempted to either confuse or deepen our understanding of aromatic compounds. There is a lot to mention in this regard, and I will focus on one area that was not covered in the Angewandte article – the Mills-Nixon effect. Chemists really struggled with benzene for a long time; the gist of the argument was whether or not Kekule’s structure corresponded to an equilibrating mixture of two cyclotrienes. Pauling’s suggestion of resonance put it all to rest, but for a while the research community was putting forth proposals that substituents on benzene could dramatically affect bonding by shifting the equilibrium... Who can blame them? After all, if there is an equilibrium, Le Chatelier’s principle should apply. The Mills-Nixon effect hails from those days and amounts to a suggestion that annelated benzenes such as the one you see on the right hand side below ought to be cyclohexatriene-like, with double bonds positioned exocyclic to the five-membered rings. All of this later turned out to be wrong – there is no Mills-Nixon effect and experimentally observed differences in bond lengths are much smaller than what is to be expected of a true cyclohexatriene-like structure. Jay Siegel did some really nice work in this regard and you can read about it in the reference below. Coming back to the origins of the Mills-Nixon effect, the same set of arguments (equilibrium or not) later haunted chemistry during the non-classical ion years. Our science is very cyclical and the root causes of all controversies are suspiciously similar.



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